1. Field of the Invention
This invention relates to methods of, and apparatus for, eye surgery, and more particularly to a laser-method and apparatus for corneal and intraocular surgery.
2. Related Art
The concept of correcting refractive errors by changing the curvature of the eye was initially implemented by mechanical methods. These mechanical procedures involve removal of a thin layer of tissue from the cornea by a microkeratome, freezing the tissue at the temperature of liquid Co.sub.2, and re-shaping the tissue in a specially designed lathe. The thin layer of tissue is then re-attached to the eye by suture. The drawback of these methods is the lack of reproducibility and hence a poor predictability of surgical results.
With the advent of lasers, various methods for the correction of refractive errors and for general eye surgery have been attempted, making use of the coherent radiation properties of lasers and the precision of the laser-tissue interaction. A CO.sub.2 laser was one of the first to be applied in this field. Peyman, et al., in Ophthalmic Surgery, vol. 11, pp. 325-9, 1980, reported laser burns of various intensity, location, and pattern were produced on rabbit corneas. Recently, Horn, et al., in the Journal of Cataract Refractive Surgery, vol. 16, pp. 611-6, 1990, reported that a curvature change in rabbit corneas had been achieved with a Co:MgF.sub.2 laser by applying specific treatment patterns and laser parameters. The ability to produce burns on the cornea by either a CO.sub.2 or a Co:MgF.sub.2 laser relies on the absorption in the tissue of the thermal energy emitted by the laser. Histologic studies of the tissue adjacent to burn sites caused by a CO.sub.2 laser reveal extensive damage characterized by a denaturized zone 5-10 .mu.m deep and a disorganized tissue region extending 50 .mu.m deep. Such lasers are thus ill suited for eye surgery.
In U.S. Pat. No. 4,784,135 Blum et al. discloses the use of far-ultraviolet excimer laser radiation of wavelengths less than 200 nm to selectively remove biological materials. The removal process is claimed to be by photoetching without using heat as the etching mechanism. Medical and dental applications for the removal of damaged or unhealthy tissue from bone, removal of skin lesions, and the treatment of decayed teeth are cited. No specific use for eye surgery is suggested, and the indicated etch depth of 150 .mu.m is too great for most eye surgery purposes.
In U.S. Pat. No. 4,718,418, L'Esperance, Jr. discloses the use of a scanning ultraviolet laser to achieve controlled ablative photodecomposition of one or more selected regions of a cornea. According to the disclosure, the laser beam from an excimer laser is reduced in its cross-sectional area, through a combination of optical elements, to a 0.5 mm by 0.5 mm rounded-square beam spot that is scanned over a target by deflectable mirrors. To ablate a corneal tissue surface with such an arrangement, each laser pulse would etch out a square patch of tissue. An etch depth of 14 .mu.m per pulse is taught for the illustrated embodiment. This etch depth would be expected to result in an unacceptable level of eye damage.
Another technique for tissue ablation of the cornea is disclosed in U.S. Pat. No. 4,907,586 to Bille et al. By focusing a laser beam into a small volume of about 25-30 .mu.m in diameter, the peak beam intensity at the laser focal point could reach about 10.sup.12 W/cm.sup.2. At such a peak power level, tissue molecules are "pulled" apart under the strong electric field of the laser light, which causes dielectric breakdown of the material. The conditions of dielectric breakdown and its applications in ophthalmic surgery had been described in the book "YAG Laser Ophthalmic Microsurgery" by Trokel. Transmissive wavelengths near 1.06 .mu.m and a frequency-doubled laser wavelength near 530 nm are typically used for the described method. Near the threshold of the dielectric breakdown, the laser beam energy absorption characteristics of the tissue changes from highly transparent to strongly absorbent. The reaction is very violent, and the effects are widely variable. The amount of tissue removed is a highly non-linear function of the incident beam power. Hence, the tissue removal rate is difficult to control. Additionally, accidental exposure of the endothelium to the laser beam is a constant concern. This method is not optimal for corneal surface or intraocular ablation.
An important issue that is largely overlooked in all the above-cited references is the fact that the eye is a living organ. Like most other organs, eye tissue reacts to trauma, whether it is inflicted by a knife or a laser beam. Clinical results have shown that a certain degree of haziness develops in most eyes after laser refractive surgery with the systems taught in the prior art. The principal cause of such haziness is believed to be roughness resulting from cavities, grooves, and ridges formed while laser etching. Additionally, clinical studies have indicated that the extent of the haze also depends in part on the depth of the tissue damage, which is characterized by an outer denatured layer around which is a more extended region of disorganized tissue fibers. Another drawback due to a rough corneal surface is related to the healing process after the surgery: clinical studies have confirmed that the degree of haze developed in the cornea correlates with the roughness of the stromal surface.
The prior art also fails to recognize the benefits of ablating eye tissue with a laser beam having a low energy density. A gentle laser beam, one that is capable of operating at a lower energy density for a surgical procedure, will clearly have the advantage of inflicting less trauma to the underlying tissue. The importance of this point can be illustrated by considering the dynamics of the ablation process on a microscopic scale: the ablation process is basically an explosive event. During ablation, organic materials are broken into their smaller sub-units, which cumulate a large amount of kinetic energy and are ejected away from the laser interaction point at a supersonic velocity. The tissue around the ablated region absorbs the recoil forces from such ejections. The tissue is further damaged by acoustic shock from the expansion of the superheated plasma generated at the laser interaction point. Accordingly a shallower etch depth or smaller etch volumes involves less ejected mass and acoustic shock, and hence reduces trauma to the eye.
It is therefore desirable to have a method and apparatus for performing eye surgery that overcomes the limitations of the prior art. In particular, it is desirable to provide an improved method of eye surgery which has accurate control of tissue removal, flexibility of ablating tissue at any desired location with predetermined ablation depth or volume, an optically smooth finished surface after the surgery, and a gentle surgical beam for laser ablation action. The present invention provides such a method and apparatus.
In many cases, the procedures proposed have been performed in the past, but they have been accompanied by inaccuracy, trauma or ocular damage. In others, they have never been capable of performance because of surgical or technical considerations.
U.S. Pat. No. 4,309,998, Aron nee Rosa et al., issued Jan. 12, 1982, described the process of optical breakdown and photodisruption, whereby tissues, transparent or not, to a given wavelength of laser radiation can be excised or ablated by sharply focusing the beam at a specific point in the tissue while achieving a local power density at the site above the threshold (greater than 10.sup.12 Watts/cm.sup.2) for optical breakdown, a complex process involving ionization, plasma formation, and mechanical disruption by secondarily propagated waves. In this patent, the inventors used a YAG laser, emitting at 1064 nm, with pulse widths in the range of 20-400 ps and energies in the range of 2-5 mJ to ablate opacities from the lens of the eye open posterior lens capsules, and cut vitreous membranes.
In their publication "Ophthalmic Neodymium YAG Lasers", Keates et al. describe the basic principles underlying photodisruption with lasers. The definition of power density is given as the ratio of beam energy in Joules divided by pulse length in second times focal spot area in square centimeters. Thus, the shorter the pulse length or the smaller the spot, the greater the power density, which is the determinant in achieving optical breakdown, whose threshold is given as 10.sup.12 W/cm.sup.2. Also, it is described that high pulse power and low pulse energy are preferred for cutting or perforating tissue, and that low pulse power and high pulse energy are associated with thermal and biophysical damage mechanisms. By using shorter pulses, an appropriate power density can be achieved in any tissue with a lower energy level, which reduces shock waves and adjacent tissue damage.
U.S. Pat. No. 4,907,586, Bille and Brown, issued Mar. 13, 1990, describes the use of the photodisruptive process for corneal and other eye surgery. In this patent, a quasi-continuous picosecond pulse width laser is used to create optical breakdown in various tissues. The inventors describe, in general, the categories of procedures that may be attempted with such a laser.
One of us (Shui T. Lai) has described technology for producing laser pulses in the femptosecond range, which, as based on the above discussion, allows high power densities to be achieved at much lower energy levels than any described in the art. Experimentally, we ablated tissue by photodisruption at various pulse widths and energy levels and have demonstrated the attainment of superior results with respect to the procedures described herein when operating in the femptosecond range as opposed to the picosecond range, with respect to pulse width. Light and electron microscopy have clearly demonstrated less adjacent damage, sharper incisions, and the ability to more accurately localize the surgical interaction, which is mandatory for optical success.
Corneal operations are typically performed for either therapeutic or optical considerations. In the therapeutic class are such procedures as lamellar keratoplasty and penetrating keratoplasty or corneal transplantation. The classic operation of lamellar keratoplasty is designed to remove scarred, irregular or opaque corneal tissue from across the visually critical central optic zone of the cornea and replacement with a partial donor cornea to restore the corneal shape and clarity, thereby improving vision. It relates also to other operations on the anterior cornea designed primarily to produce changes in the optical imaging of the cornea, thereby correcting optic errors of the eye, such as myopia (nearsightedness), hyperopia (farsightedness), astigmatism, optical aberrations and combinations thereof.
The operation of partial thickness lamellar keratoplasty to remove corneal opacities has been practices for many years (see Brightbill, F S, Corneal Surgery, Chapter 33, C. V. Mosby Co., St. Louis, 1986). It has classically been performed by direct mechanical removal of a circular disc of tissue of constant thickness and replacement thereof with a similarly shaped piece of donor corneal tissue. The optical quality of the final cornea has frequently been known to be irregular or with some interface opacity, and often resulted in reduction of vision from normal. More recently, this procedure has been performed with a high-speed mechanical microkeratome to effect detachment of the anterior disk both from the patient's cornea and from the donor's cornea. However, this technique is technically difficult and accompanied by incomplete or irregular resections, resections of inappropriate depth, and can cause penetration into the eye.
In penetrating keratoplasty, a partial diameter but full-thickness section of the patient's cornea is removed and replaced with a donor corneal button similar in size. Typically, the walls of the incision are vertical, or parallel to the visual axis. This provides a button whose walls are a portion of a cylinder. Also, the transplant button is typically round, as this facilitates use of mechanical trephines. However, many surgeons believe that round buttons may not be ideal for several reasons, involving healing, rejection, and endothelial cell population transplanted. However, there is no technique at present to allow for cutting corneal buttons of any shape, with accuracy. Also, some surgeons believe that walls other than vertical may decrease various complications, such as wound leak, astigmatism, etc. This is not accomplishable by any means.
This operation is notoriously accompanied by high astigmatism following the surgery, which limits its success, and which is believed to result from the healing process of irregularly cut wound margins. Typically, some trephine blade is used to make most of the incision, which is then completed manually with scissors. Surgeons have long sought an ideal way to cut corneal tissue as evidenced by the number of different trephines developed.
Corneal surgery is also performed frequently for modifying the optical or refractive power of the eye. Such operations fall into different categories or approaches. They include the lamellar techniques, whereby corneal tissue is removed from within the cornea, leaving its anterior outermost structure intact, incisional techniques, whereby cuts are made through the anterior surface and to deep in the cornea, thereby causing secondary compensatory changes in the curvature of the anterior cornea, and, most recently, direct ablation or removal of the anterior cornea in a controlled fashion, using an ultraviolet laser, to produce a new surface with different curvature.
Barraquer teaches the general art of altering the anterior corneal curvature of the eye to effect changes in refraction, or optical imaging of the eye, with the operation of keratomileusis, a form of lamellar kerotoplasty. (See IBID, Chpt. 37). In this procedure, a circular lamellar disc of constant thickness centered on the visual axis is removed from the front of the patient's cornea with a high-speed mechanical microkeratome. Following said removal, called a lamellar keratectomy, the resected lamellar disc of constant thickness is placed onto one of two available devices (Barraquer cryolathe, BKS device) to effect modification in shape to produce a lenticule with refractive optical power. Although operationally different, both devices effect the production of a refractive corneal lenticule. The lenticule is produced by volumetric mechanical removal of stromal tissue from the cut and exposed corneal stromal surface of the resected lamellar disc. Such tissue removal may be greatest in the center of the disc, which allows for correction of myopia, or toward the outer periphery, which allows for correction of hyperopia. In any event, the tissue removal is usually such that there is a smooth and regular transition of thickness as one traverses the optically modified (optic zone) area. Following tissue removal from the disc (now called a lenticule), it is replaced onto the patient's cut stromal surface remaining behind after the initial keratectomy. Said replacement results in a new anterior corneal curvature and alteration in the optic imaging of light by the cornea.
In addition, the mechanical microkeratome has also been used in two recently developed lamellar refractive procedures--in-situ keratomileusis and hyperopic lamellar keratectomy. In in-situ keratomileusis, the microkeratome is used to detach a thin slice of constant thickness from the patient's cornea. This is followed by a second keratectomy with the microkeratome, smaller in diameter than the first and concentric within the confines of the first. The initially resected disc is then replaced atop the bed. The second keratectomy also resects a disc of parallel faces or constant thickness. When the cap is replaced, it drops into the cavity, thereby causing flattening of the anterior cornea. The degree of flattening and optical correction is related to the depth and diameter of the second resection. This procedure is to be distinguished from classic keratomileusis performed with a laser for making the second, or optical, ablation, as the physical process is different and in-situ keratomileusis provides a larger functional optical zone, greater refractive correction and greater stability.
In hyperopic lamellar keratectomy a lamellar disc of corneal tissue is deeply removed and simply replaced to effect a correction of hyperopia. The amount of correction is related to the diameter of the resected disc. However, experience has shown that the mechanical microkeratome has the potential of becoming stuck in its passage across the eye, of producing a surface of irregular or poor quality and of being inaccurate with respect to diameter and thickness, all of which compromise the result obtained. Also, there is technical difficulty in using the microkeratome, despite its being automated recently.
Another category of corneal procedures consists of incisional techniques. Here, as in radial keratotomy, for example, the surgeon makes a series of incisions deep into the cornea, sparing a central incision free zone. This results in flattening of the cornea and correction of myopia. Or, various other geometric patterns of incisions may be made, such as transverse or arcuate. These are typically used to correct astigmatism. The major limitation with these techniques is the inability of the surgeon to cut precisely in the desired location and in a reproducible fashion, especially with respect to the length, depth and perpendicularity of the incisions. This results in a lack of reproducibility and inaccuracy. It is well established that incisions too short lead to undercorrection, too long to overcorrection and bothersome visual symptoms, and deviation from perpendicularity, or oblique incisions, like shallow incisions, to undercorrection and instability. Thus, the inability to incise corneal tissue in a reproducible manner, according to plan, has been a major drawback.
A characteristic of the foregoing discussion of operative procedures is that they all attempt to spare the anterior central corneal structure from surgical damage. This is in contrast to the recently developed excimer laser surgical procedure where the anterior surface is progressively ablated and destroyed by the surgery. U.S. Pat. No. 4,732,148, L'Esperance, issued Mar. 22, 1988, discloses a method of applying ultraviolet radiation to the anterior cornea (photorefractive keratectomy) in order to correct the optical errors of myopia, hyperopia, and astigmatism. Unfortunately, the delicate anterior membrane complex of the patient's cornea (primarily Bowman's membrane) is destroyed in the process of removal, leaving a cornea which is anatomically, and perhaps physiologically, abnormal. In addition, two other drawbacks of this method have been noted. The first is the production of haze within the operated cornea, which can be permanent, and which may be associated with visual symptoms or reduction in vision. Haze has been attributed to acoustic shock waves, induced by the laser beam of high energy, propagated in the cornea. Second, the anatomically abnormal cornea develops a healing response such that the outermost epithelial layer, regenerated over the operated area from peripheral unoperated epithelium, frequently demonstrates hyperplasia or thickening postoperatively. This can cause gross inaccuracy or instability of the obtained optical result. Also, pain and delayed rehabilitation along with the long-term use of medications that may cause glaucoma are needed. For this reason, surgeons seek procedures capable of correcting optical errors of the eye while sparing the anterior central cornea.
U.S. Pat. No. 4,903,695, Warner et al., issued Feb. 27, 1990, disclosed a method of performing Barraquer's classic keratomileusis operation using an ultraviolet or infrared laser to effect the tissue modification step, thereby replacing the cryolathe and BKS device. It also circumvents laser damage to the anterior cornea. However, the method requires the use of a mechanical microkeratome to first detach a circular disc of tissue from the patient's cornea, which has limitations as previously described. Following this mechanical cut, the laser irradiation is applied selectively to the cut stromal surface left behind on the patient to remove tissue in a controlled fashion such that when the initially resected disc is replaced onto the bed from which it was removed, a new curvature is imposed onto the anterior corneal surface. In this teaching, the "predetermined curvature" imposed on the ablated corneal bed is intimately related to the final desired anterior curvature of the cornea, and is in fact equal to the final radius of curvature desired minus the thickness of the disc resected by the microkeratome. It is important to note that radiation is applied to resect from the bed, as clearly described in the drawings, a lens of optical power from the cornea, not to create a trough of constant depth, as is the basis of in-situ keratomileusis, which induces a curvature change by a different physical process.
The corneal techniques described above have met with several shortcomings, as partly described. These consist in the inability to precisely and reproducibly cut living corneal tissue with a minimum of trauma to the cornea. The mechanical and manual techniques best exemplify this limitation. When cutting is performed with lasers, the results thus far using existing technology have not demonstrated the ability to cut with minimal trauma and maximal control. One observes a tissue interaction zone which is too large for precise tissue removal, no cutting at all if the power density is below the threshold, or adjacent tissue damage from the energy levels used.
Lasers have been used in glaucoma surgery for years. There are two major classes of glaucoma, open angle and closed angle. In open angle glaucoma, there is difficulty in the eye's intraocular fluid exiting from the eye, thus raising the pressure within the eye, causing glaucoma. The basic problem is the outflow channel for fluid, which does not function well. For this reason, a number of operations have been developed to provide increased outflow. In closed angle glaucoma, the problem lies more in the internal structure of the eye, where various abnormalities allow the pressure to build up behind the iris of the eye, causing a displacement, which in turn compresses or occludes the outflow channel, which is more or less normal. Thus, operations to correct open angle glaucoma are geared to providing outflow from the eye, whereas closed angle glaucoma is corrected by making a full-thickness hole through the iris to allow the free movement of fluid from the back of the eye, where it is made, to the front of the eye, from where it exits the eye.
Typically, two surgical approaches have been utilized. The oldest and most common is utilizing a laser such as an argon laser to achieve the desired effects in trabeculoplasty and iridotomy by photocoagulation, or the vaporization of tissue. In filtration procedures such as trabeculoplasty, minor restructuring of the tissue with partial or complete penetration is accomplished by thermal effects of the laser, thereby causing a change in structure of the trabecular meshwork, which is part of the outflow system of the eye. Surgery alters the structure such that the intraocular fluid can escape from the eye more easily, thereby reducing the eye's pressure in glaucoma patients. Also, the operation of sclerostomy is similarly performed, either from within or without the eye, to create a drainage channel for the intraocular fluid. Also, the operation of trabeculectomy is another modification, whereby a partial thickness flap of the eye's wall, the sclera, is opened, a drainage opening made into the eye itself, and the flap replaced as a partial thickness protector for the interior of the eye. Recently, lasers such as the holmium, an infrared laser, have been used in attempts to provide some of these perforations.
In closed angle glaucoma, an iridotomy, or total penetration through the iris is desired. In addition to using photocoagulation or photovaporization, one can also use photodisruption. The disadvantages of all these techniques are ocular trauma, especially in photodisruption, where each laser pulse produces a significant shock wave that can damage the delicate intraocular structures.
Lasers have also been used in cataract surgery for some time. This includes both for excising the posterior lens capsule, when opacified, and for delivering energy to the lens itself for ablating its interior substance, both to create an opening at the visual axis or to shorten the time of secondary phacoemulsification surgery by first liquefying the interior lens substance, which uses ultrasonic energy to remove the lens material and which is traumatic to the posterior lens capsule and intraocular structures. It is felt that by first ablating the lens (phacoablation) that the time needed for the ultrasound will be reduced. Unfortunately, the lasers proposed thus far, Nd:YAG and Nd:YLF have a considerable acoustic shock component in themselves at the energy levels used. When used to open the posterior capsule of the lens to remove an opacity, shock waves from the currently used Nd:YAG lasers have been shown to be able to cause complications in the posterior eye. Cataract surgery, as presently performed, requires the opening of the anterior capsule of the lens to allow the surgeon access to the lens itself. Currently, no laser has the control and gentleness of beam that allows a smooth and regular opening, or capsulorhexis. To date, this opening has been created manually, with a needle, and this can result in complications if torn irregularly or inappropriately.
Behind the lens, or in the posterior segment of the eye, lies the vitreous humour and retina. Currently, lasers utilizing photodisruption can be used to cut or ablate membranes within the vitreous cavity, but only with significant shock and only at a safe distance from the retina. Surgeons seek a fine cutting laser with minimal shock wave that can allow membranes close to the retina to be resected.
Also, many patients have retinal pathology, such as sub-retinal membranes or blood vessels. In most instances, these conditions are not treatable as they would cause substantial damage to vision, as the functional part of the retina overlies the abnormal pathology, and attempts to remove or ablate the pathology lead to destruction of the overlying retina. A laser capable of ablating behind the retina with minimal thermal or shock component would allow many patients a surgical option for cure of their blindness.